The subject invention relates to a fastener having a non-circular, treaded shank. Fasteners having threaded shanks include bolts for engaging nuts and screws for forming mating threads when driven into a round hole. It is known in the art, that the threaded shank must have a round crossection for proper thread to thread engagement with a nut or tapped hole A round crossection can be circular but does not as a necessity need to be circular. A, constant shank breadth (diameter) of a round shank is required for proper thread to thread engagements but a constant shank radius (measured from point O in FIG. 2) is not. In fact, for many fastener applications, a non-circular crossection is preferred to a circular crossection provided that the diameter is constant. A shank crossectional geometry having three evenly spaced lobes meets this requirement and is presently known in the art.
The introduction of lobes to a fastener shank crossectional geometry provides enhanced locking benefits particularly for a screw forming mating (nut) threads on the inner surface of a round hole. However, a Three lobe shank has inherent limitations. The three lobe geometry substantially reduces the crossectional area of the shank in manufacturing the lobular crossection by extrusion from circular raw material, thereby reducing overall shank strength. Further, the threads on a three lobe shank have less contact area with the threads on a nut, which can lead to thread shearing resulting in the loss of engagement with the nut. Still further, the three lobe geometry for a thread-forming screw is known, in some instances, to cause unnecessary damage to the substrate of an untapped hole because of the relatively large variation in radius around the circumference. Still further, a three lobe shank is difficult to manufacture because of a significant reduction in crossection area of the shank that is required for forming the three lobe geometry by extrusion, starting with a circular crossection.
The present invention discloses a threaded fastener which includes a head and a threaded shank. The shank includes a helical rib forming a thread, wherein the shank includes a generally polygonal crossection defining a constant breadth. The generally polygonal shape includes at least five lobes separated by at least five intermediate convex surfaces.
The advantage of a five lobe design over a three lobe design is due to the increase in the crossectional area which yields a proportionately increased shank tensile strength. Additionally, the increased crossectional area increases the thread to thread contact area between mating external and internal threads, which reduces the potential for thread shear-out. The five lobe shank reduces the amount of damage caused to the substrate defining a hole into which threads are being formed. Finally, the five lobe shank requires less reduction in shank crossectional area during the manufacturing process, which improves manufacturing ease.